1. Field of the Invention
The invention relates to a polymer film having a helical molecular structure with adjustable pitch gradient, to a process for its production, and to its use.
2. Description of the Related Art
Many optical applications utilize films which convert unpolarized light to polarized light by absorbing one polarization component of the light. In the ideal case, a maximum of 50% of the light is allowed to pass through such a polarization film. In addition to the loss of light yield, this process has the further disadvantage that, especially in the case of bright light sources, the absorption results in intense heating of the polarizer. Both disadvantages are avoided in the case of reflective polarizers which reflect the fraction of the undesired polarization back into the light source. Depending on the particular arrangement of the optical components of the light source, this fraction may change its polarization to the desired polarization by reflection or scattering and thus contribute at least partly to an increase in the light yield.
One example of a reflective polarizer is described in U.S. Pat. No. 5,235,443, in the form of films of cholesteric liquid crystals. Cholesteric liquid crystals are substances having a helical alignment of molecules. A thin layer thereof may be prepared between two suitable substrates in such a way that the helix axis is at right angles to the substrate surfaces. The pitch of the helix is material-dependent and is constant over the layer thickness. Such a layer reflects a circular light component virtually fully when sense of rotation and wavelength λ of the light in the material coincide with the sense of rotation and the pitch p of the cholesteric helix and the layer thickness is a multiple of the pitch (cholesteric reflection). In contrast, the second circular light component having opposite sense of rotation, and light components having other wavelengths are, in the ideal case, allowed to pass through fully. When required by the application, the circular-polarized light fraction may be converted to linear-polarized light by an additional quarter-wave retardation layer.
Cholesteric reflection occurs in a spectral band between two wavelengths λ1=p·no and λ2=p·ne where ne and no denote the extraordinary and ordinary refractive index of the material, respectively. This band is characterized by the center wavelength λ0=p·n which depends upon the average refractive index n=(no+ne)/2 and upon the pitch p of the material, and the width Δλ=p·Δn which is determined at a given pitch p by the birefringence Δn=ne−no. In practice, the birefringence of most cholesteric materials in the visible spectral region is restricted to values of less than 0.3. From this follows a maximum possible bandwidth of about 100 nm, although usually only bandwidths of from 30 nm to 50 nm are achieved. The intensity of the light reflected in the cholesteric band increases with the number of pitches λ0/n in the layer, and for unpolarized incident light, achieves the maximum value at 50% of the incidence intensity. Only above a layer thickness of about three pitches can reflection be observed. The minimum layer thickness required for most cholesteric materials in the visible spectral region is therefore a few μm.
A prerequisite for the use of liquid-crystalline materials in applications such as a reflective polarizer or as LC pigments is sufficient thermal and mechanical stability of the helical molecular structure. This stability may be achieved by fixing the state of alignment by polarization or by rapid cooling to temperatures below the glass point. Such stable cholesteric layers are described, for example, by R. Maurer et al. in “Polarizing Color Filters made from Cholesteric LC Silicones” in SID 90 Digest, 1990, p. 110-113.
In addition to application as a polarization film, the literature describes additional applications of cholesteric films in various optical elements which will not be further detailed here. In all applications, the center wavelength λ0 and the width Δλ of the cholesteric band should be adjusted precisely. For specific applications as a reflective polarizer, which are used, for example, to improve brightness of an LCD, it is especially necessary that the reflection band covers the entire visible spectral region, i.e. that the bandwidth Δλ should be more than 300 nm. However, the reflection properties of such films and in particular the polarization of the light allowed to pass through depend on the viewing angle relative to the normal on the film. The band of a reflective polarizer should cover at least the region from 450 nm to 600 nm for all relevant viewing angles. In order to compensate for the effect of color shifting as a function of the viewing angle which is described by S. Ishihara et al. in “Preparation and properties of optical notch filters of cholesteric liquid crystals” in POLYMER, Vol. 29, 1988, p. 2141-2145, the band should therefore, for example, range from at least 450 nm to 850 nm for a desired viewing angle range of up to 45° from a right angle to the surface of the display. Moreover, the films should additionally be very thin, in order to minimize the viewing angle dependence of the polarization. For broadband LC pigments which, for example, can generate special color effects in the visible region, lower bandwidths of around 100 nm are also required—however, these should be achieved in layers having a thickness of less than 6 μm.
One way of producing polymer films having a bandwidth Δλ which is greater than the λ0·(ne−no)/n value corresponding to the liquid-crystalline material as described by Maurer et al. is to provide an optical element composed of a plurality of cholesteric layers having different center wavelengths. However, this method is very cost-intensive and has the disadvantage that the optical quality decreases with every additional layer as a consequence of scattering at defect sites and inhomogeneities. In addition, this process cannot be employed for LC pigments, because total thicknesses of below 6 μm are difficult to realize using a plurality of thinner individual layers. A more suitable method for producing a broadband cholesteric polarizer is to replace the sequence of individual layers having constant pitch of the helical molecular structure by a single layer having continuously increasing or decreasing pitch. The broadening of the reflection band by virtue of a gradient in the pitch of the helix (pitch gradient) has been known for some time from theoretical investigations (for example S. Mazkedian, S. Melone, F. Rustichelli, J. Physique 37, 731 (1976) or L. E. Hajdo, A. C. Eringen, J. Opt. Soc. Am. 36, 1017 (1979)). According to the current state of the art, there are several industrial processes which enable a layer having a pitch gradient to be produced. They can be divided substantially into the following three categories:                1. generation of a crosslink density gradient and diffusion of the uncrosslinked components,        2. lamination of at least two layers having different chemical compositions, and subsequent diffusion, and        3. depth-dependent extraction of nonpolymerized fractions from a semipolymerized film.Each of these processes requires specific material mixtures which are formulated for the particular process.        
European patent application EP 0 606 940 A2 describes a process in which a mixture of chiral and nematic monomers having different reactivity with regard to their polymerization properties is polymerized over a prolonged period with a low UV dose, so that diffusion of the monomers occurs during the polymerization and then generates a pitch gradient as a consequence of the mixed material composition. The driving force for the diffusion is a gradient in the crosslinking density which is caused by a gradient in the UV intensity in the material. The reduction in the UV intensity in the film is controlled by adding a dye, which, however, has disadvantages for the stability and the optical properties of the film. Owing to the requirement that the film finally has to be fully polymerized in order to achieve good mechanical stability, a high UV absorption is not possible and therefore the UV gradient in the film is not particularly steep. This makes the overall process relatively slow, and thus the process has only limited suitability for industrial preparation of a broadband cholesteric polarizer on a continuous substrate, for example a plastics film. The weak UV gradient also has an effect on the pitch gradient, so that the minimum thickness of the layer for a required width of the reflection band is not sufficient for applications in which thin layers of below 10 μm are required. In the examples of EP 0 606 940 A2, the layer thicknesses of the films are 20 μm.
The process described in European patent application EP 0 982 605 A1 also utilizes absorption of the UV radiation in a cholesteric layer in order to generate a pitch gradient. In the LC mixtures used, a transition from the cholesteric to a smectic phase proceeds during the polymerization and brings about untwisting of the helical structure. This process is potentially more rapid than a pure diffusion process, although broadband films are not achieved in the examples cited below an irradiation time of 30 s. A further disadvantage of this method is that the realignment of the mesogens into the smectic phase leads to the formation of small domains. The light scattering caused by the domain borders reduces the degree of polarization of the light allowed to pass through.
A further process described in European patent application EP 0 885 945 A1 overcomes the disadvantage of the slow diffusion process caused by the weak crosslinking density gradient in the process of EP 0 606 940 A2, by dividing the UV irradiation procedure into two steps between which a change in the temperature of the film takes place. In this method, the thermochromic property of the cholesteric phase is utilized and leads to a rapid realignment of the mesogens and therefore to a more rapid setting of the pitch in the zones of the film having different degrees of crosslinking. However, a disadvantage of this process is that the necessary bandwidth of above 300 nm is difficult to achieve under the necessary conditions for industrial production.
U.S. Pat. No. 6,099,758 A claims a process in which the crosslinking density gradient is increased by a further gradient occurring in addition to the UV gradient by virtue of the inhibiting influence of the substrates used. Although this process is more rapid than that described in EP 0 606 940 A2, broadband films are not achieved in the examples cited below an irradiation time of 30 s. Owing to the low total UV doses which are obtained from the specified irradiation times and UV intensities, the thermal and mechanical stability of the films are low. The production process becomes technically complicated in that the cholesteric film either has to be produced between two different substrates, or in that, when a single substrate is used and polymerization is effected in an air atmosphere, an additional oxygen barrier layer on the substrate is necessary.
Likewise described in EP 0 606 940 A2 is a process in which two layers having different chemical compositions, of which one has cholesteric alignment, are brought into contact. The cholesteric layer is swollen by diffusion so that a pitch gradient results. Finally, the film is polymerized. In another embodiment of this process in the European laid-open specification EP 0 881 509 A2, two cholesteric films having different pitch are laminated and melted by controlled diffusion at elevated temperature in such a way that a continuous transition between the two pitches results. A disadvantage of this process is the complicated production of at least two different layers and the technically difficult control of the diffusion process between the two layers.
European patent application EP 0 875 525 A1 discloses a semipolymerized cholesteric films on a substrate, prepared by immersing the coated substrate in baths of organic solvents which bring about extraction of nonpolymerized fractions in the film. Since this effect has the greatest intensity at the surface of the film, depth-dependent shrinkage and thus a pitch gradient are generated after drying and heat-treating the film. In order to obtain reproducible results, large amounts of solvents have to be constantly exchanged, which is undesirable from the environmental protection point of view. The necessary purification of these solvents and the complicated process control additionally increase the costs considerably.
In the existing patent literature, there is discussion with sometimes contradictory results as to how a cholesteric polarizer prepared by the above-described processes has to be aligned in an LCD in order to achieve a very high light yield with very low dependence upon the viewing angle. In the case of the arrangement of the cholesteric polarizer between the light source and the electrically switchable liquid crystal cell, it is possible that, in an asymmetric film having continuous pitch gradient, which has previously been described in the existing examples as a constant gradient, or, in the mathematical sense, as a monotonously rising or falling gradient, for either the side having shorter pitch or the side having longer pitch to face in the direction toward the light source. In the case of the symmetrical film hypothetically described in U.S. Pat. No. 6,099,758 A, there is no preferred direction.